1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic type or electrostatic recording type such as a copying machine, a page printer and the like, and more particularly, it relates to a developing apparatus, and a process cartridge and an image forming apparatus which have such a developing apparatus.
2. Related Background Art
In such developing apparatus, since charging ability for toner is very good in an electrophotographic process, a developing apparatus using two component developer developing agent including toner and carrier (referred to as "two-component developing apparatus" hereinafter) has widely been used.
FIG. 5 is a sectional view showing a conventional two-component developing apparatus.
As shown in FIG. 5, the two-component developing apparatus has a developer container 110 containing two-component developer, and a developing sleeve 102 as a developer bearing member.
The developing sleeve 102 is a rotatable hollow metallic sleeve in which a magnet roller (magnetic field generating means) 103 is fixed and which serves to bear the developer in the developer container 110.
An A screw (first agitating means) 105 having an axis substantially parallel with an axial direction of the developing sleeve 102 is disposed within the developing container 110 and serves to convey and agitate the developer in the developer container 110 in a direction shown by the arrow in FIG. 8.
Further, a B screw (second agitating member) 106 is disposed at an opposite side of the developing sleeve 102 with respect of the A screw 105.
FIG. 8 is a top view of the two-component developing apparatus shown in FIG. 5.
As shown in FIG. 8, the A screw 105 and B screw 106 are arranged so that their axes become substantially parallel with each other, and the developer container 110 is partitioned by an inner wall (partition member) 107 for dividing the interior of the developer container 110 into two chambers, which inner wall is provided at its both ends (in the axial direction of the developing sleeve 102) with openings for permitting the developer to move between the A screw 105 and the B screw 106.
Further, since the A screw 105 and B screw 106 convey the developer in opposite directions, a circulating path for circulating developer without discontinuity is defined within the developer container 110.
The developer container 110 is provided at its upper part with a replenishing port 108 for replenishing the developer in the developer container 110.
When toner (developer) density in the developer is decreased as a result that the toner is consumed during image formation, a proper amount of toner is replenished in the developer container 110 through the replenishing port 108 by means of a toner replenishing mechanism 109 so that the toner density of the developer is always maintained to a constant value.
However, in such a developing apparatus, it is very difficult to design the developing apparatus so that the developer is well circulated, and, thus, it is desirable that the two-component developing apparatus satisfies the following three conditions or requirements.
First, it is preferable that a surface of the developer within the developer container near the A screw (referred to as "agent surface" hereinafter) is above an uppermost part of the A screw.
The reason is that, since the A screw 105 is positioned near the developing sleeve, as shown in FIG. 6, if the agent surface is lowered below the uppermost part of the A screw, the developer supplied to the developing sleeve between uneven (i.e., increased or decreased) in dependence upon a pitch of vanes of the A screw, with the result that density unevenness of an image may occur due to the pitch of the screw (referred to as "screw pitch unevenness").
So long as the agent surface near the A screw in the developer container is positioned above the uppermost part of the A screw, since the developer is supplied to the developing sleeve relatively uniformly, the screw pitch unevenness is hard to generate.
Second, it is preferable that an agent surface near the B screw in the developer container is below an uppermost part of the B screw.
The reason is that, the side of the B screw in the developer container has a purpose for agitating the developer in the developer container, and, thus, if the agent surface is increased above the uppermost part of the B screw, the agent above the B screw is hard to be agitated.
Particularly, as shown in FIG. 7, if the toner is replenished when the agent surface near the B screw is above the uppermost part of the B screw, the toner having specific weight smaller than the developer may be floating on the agent surface.
In such a case, the toner is hard to be mixed with the developer, with the result that non-charged toner is supplied to the developing sleeve, thereby causing fog or poor density.
However, in the case where the agent surface near the B screw within the developer container is below the uppermost part of the B screw, even when the toner is replenished, the toner is semiforcibly entered into the developer by rotation of the B screw to be agitated adequately, thereby preventing the fog and the poor density.
Third, the agent surfaces near the B screw and the A screw within the developer container may be horizontal at their screw areas.
The reason is that, if the agent surface near the A screw within the developer container is inclined in a longitudinal direction, the amount of the developer supplied to the developing sleeve becomes uneven in the longitudinal direction, with the result that difference in density may occur along the longitudinal direction.
On the other hand, if the agent surface near the B screw within the developer container is inclined in the longitudinal direction, the charging ability due to agitation may be reduced.
In order to satisfy these requirements, it is necessary to perform delicate adjustments of the shape of the developer container, the amount of the developer, the pitch between the vanes of the screw, rotational speeds of the screws and the like.
However, if the amount of the developer container is decreased, as shown in FIG. 6, the uppermost part of the A screw will be come out from the agent surface; whereas, if the amount of the developer is increased, as shown in FIG. 7, the B screw will be lowered below the agent surface.
Further, in order to satisfy the first and second requirements, if a conveying speed of the B screw is merely increased, the upstream side (in the conveying direction) of the agent surface of the A screw will be higher than the downstream side thereof, with the result that the third requirement cannot be satisfied. Consequently, the difference in density occurs in the longitudinal direction. Thus, very delicate adjustment is requested to make the design of the developing apparatus very difficult.